1. Field of the Invention
The present invention is concerned with methods of forming large quantities of ligated nanoparticles which can be deposited in two- and three-dimensional superlattices. Broadly speaking, the method involves initially forming a first colloidal dispersion made up of nanoparticles solvated in a molar excess of a first solvent, a second solvent different from the first solvent, and a quantity of ligand moieties. Thereupon, a substantial part of the first solvent is removed and the ligand moieties are caused to ligate to the nanoparticles to give a second colloidal dispersion. Preferably, the second dispersion is subjected to a heat and reflux digestive ripening process to give substantially monodispersed colloidal particles. The invention also pertains to the ligated nanoparticle colloidal dispersions and to the final products.
2. Description of the Prior Art
It is known that high surface area nanoparticles can be formed by a vaporization-co-condensation process sometimes referred to as the solvated metal atom dispersion (SMAD) method. The latter involves vaporization of a metal under vacuum and codeposition of the metal atoms with the vapors of a solvent on the walls of a reactor cooled to 77 K (liquid nitrogen temperature). After warm-up, nanoparticles are stabilized both sterically (by solvation) and electrostatically (by incorporation of negative charge). The SMAD technique was first disclosed in 1986 by Klabunde and co-workers, and is also described in U.S. Pat. No. 4,877,647. A major advantage of the SMAD process is that no biproducts of metal salt reduction are present, and pure metal colloids are formed. Additionally, the SMAD process lends itself to industrial-scale operations, as opposed to other competing processes such as the inverse micelle and reductive procedures for metal colloid preparation.
Organization of nanoparticles into two and three-dimensional structures (nanocrystallinc superlattices, NCSs) leads to the formation of materials characterized by very different properties compared to those of the discrete species. The manifestation of novel and technologically attractive properties is due to the collective interactions of the particles, as well as to the finite number of atoms in each-crystalline core. Synthesis and characterization of such materials are interesting from both fundamental and industrial points of view. Regularly arranged nanosized particles find applications in the development of optical and electronic devices, and magnetic recording media, for example. Nanoparticles of gold and other noble metals have attracted significant attention not only because of ease of preparation, but also due to their potential application in nano and microelectronics. Heretofore the challenge has been to form a structure of a planar array of small metal islands separated by tunnel barriers for use in electronics. Gold nanoparticles are excellent candidates in this respect.
Numerous methods for synthesis of particles arranged in 2D- and 3D-NCSs have been reported in the literature. The most common procedures include reduction of a suitable metal salt in the presence of different stabilizing agents. In all methods, the most important requirement is the ability to produce monodispersed particles that can order over a long-range. Crystalline arrays of particles covered by organic molecules have become of great interest, especially since the improved synthesis of thiol-stabilized gold nanoparticles has been developed (Brust, et al., J. Chem. Soc., Chem. Commun., 1994, 801-802). Their advantage is that they behave as simple chemical compounds in respect that they can be dissolved, precipitated, and redispersed without change in properties, much as molecular crystals can.
The present invention is broadly concerned with methods of forming ligated nanoparticle colloidal dispersions and recovered ligated nanoparticles which may be in superlattice form. In general, the method involves initially forming a first colloidal dispersion comprising nanoparticles solvated in a molar excess of a first solvent, a second solvent different than the first solvent, and a quantity of ligand moieties. Next, a substantial part of the first solvent is removed and the ligand moieties are caused to ligate to the nanoparticles to give a second colloidal dispersion comprising the ligated nanoparticles solvated in the second solvent. If desired, the ligated nanoparticles may then be recovered as a dry product which, depending upon the nature of the nanoparticles and ligands selected, may assume a superlattice configuration.
Preparation of the first colloidal dispersion is preferably accomplished by vaporizing the solid substance (e.g., metal or metal salt) and first solvent in a reactor to give vaporized atoms or molecules and depositing the vaporized atoms or molecules and first solvent onto a cold surface. Upon subsequent warming of this mixture, nanoparticles are formed by aggregation of the atoms or molecules, and these nanoparticles and first solvent are allowed to mix with a second solvent and ligand moieties. Thereupon, the first solvent is removed by vacuum, which substantially completely eliminates the first solvent and also, to a limited degree, some of the second solvent.
In a particularly preferred technique, the second colloidal dispersion is subjected to a digestive ripening process so that the variation in particle size of the ligated nanoparticles is reduced; this ripening process is advantageously carried out until the second colloid is essentially monodispersed. This ripening process is also important if a superlattice dry product is desired.
The nanoparticles useful in the invention are generally selected from the group consisting of the elemental metals having atomic numbers ranging from 21-34, 39-52, 57-83 and 89-102, all inclusive, the halides, oxides and sulfides of such metals, and the alkali metal and alkaline earth metal halides. Elemental gold and silver are particularly preferred, with elemental gold being the single most preferred nanoparticle material. The nanoparticles should have an average diameter of from about 2-50 nm, and more preferably from about 3-15 nm. Similarly, the nanoparticles should have a BET surface area of from about 15-500 m2/g, and more preferably from about 50-300 m2/g.
The first and second solvents should be selected so that the first solvent may be readily removed by vacuum distillation or other techniques from the initial colloid. In practice, the first solvent should have a boiling point at least about 25xc2x0 C. (more preferably at least about 40xc2x0 C.) below the boiling point of the second solvent. Of course, the first and second solvents must also have the ability to solvate the nanoparticles and ligated nanoparticles, respectively.
Although a wide variety of solvents may be employed, preferably the first solvent is a ketone, and especially a ketone selected from the group consisting of ketones of the formula 
where R1 and R2 are independently and respectively selected from the group consisting of straight and branched chain C1-C5 alkyl and alkenyl groups, and the C1-C5 straight and branched chain alcohols. The single most preferred first solvent is acetone. The first solvent should be used at a level so that it is in molar excess relative to the nanoparticles, and preferably a molar excess of from about 50-1000 should be established.
The second solvent is preferably an aryl organic solvent such a toluene or xylene. More broadly, the solvent is advantageously selected from the group consisting of solvents of the formula 
where X1 and each X2 are each independently and respectively selected from the group consisting of H and C1-C5 straight and branched chain alkyl and alkenyl groups, n is from 0 to 3, each X2may be independently located at any unoccupied ortho, meta or para position relative to X1.
A variety of ligands may be used in the invention, and can be atoms, ions, or compounds. As used herein xe2x80x9cligand moietiesxe2x80x9d refers to all such ligand species. The preferred class of ligands are thiol compounds selected from the group consisting of compounds
R3SH
where R3 is a C5-C20 straight or branched chain alkyl or alkenyl group. More preferably, R3 is a C10-C15 straight or branched alkyl or alkenyl group; an especially preferred ligand is dodecanethiol.
The digestive ripening process comprises the step of heating and refluxing the second colloidal dispersion, preferably at a temperature of from about 60-180xc2x0 C. under an inert gas such as argon for a period of from about 10-400 minutes. The goal of digestive ripening is to reduce the particle size variation in the ligated nanoparticles; preferably, this process is carried out to achieve a ligated nanoparticle surface area of up to about 20% above and below the mean surface area of the ligated nanoparticles.
The final ligated nanoparticles in general have the formula Y(Z)x where Y is the nanoparticle and Z is the ligand; x is variable depending upon the nanoparticle and ligand selected. In the case of the preferred Au(dodecanethiol) ligated nanoparticles, x would typically A range from about 300-10,000, with a ligand density on the gold nanoparticle surface ranging from about 1-10 ligand moieties per square nanometer of nanoparticle surface area.